As shown in FIG. 1A, a driving circuit of an organic EL panel generally has a constant current source 11 and switching means SWs1-SWsm, respectively, for every data line, and a cathodic power supply potential VC and switching means SWc1-SWcn, respectively, for every scanning line, against the organic EL panel having an organic EL element PEm,n, disposed at respective crossover points of a plurality of the data lines (anodic lines SEG1-SEGm) and a plurality of the scanning lines (cathodic lines COM1-COMn). These switching means are controlled by a drive control circuit 10 and can be turned into select state or unselect state, respectively.
In common operation to cause the organic EL panel to emit light for displaying, the switching means SWcn of the respective scanning lines COMn is turned ON (connected to a grounding potential VG) and OFF (connected to the cathodic power supply potential VC) in such a manner as to have operation waveforms shown in FIG. 2 at a predetermined time interval, thereby sequentially selecting panel rows to be lighted. At this time, the switching means SWsm of the data line SEGm connected to the organic EL element PEm,n to be lighted, in the panel row selected, is turned ON, and current is supplied thereto, whereupon the organic EL element PEm,n is caused to emit light.
Since emitted light luminance of the organic EL element PEm,n is dependent on a current value, values of current supplied to the respective data lines SEGm are required to be constant values equal to each other in order to avoid display unevenness.
In order to obtain a constant current, it is desirable that the driving circuit is under small effects of its dependency on an output voltage of the constant current source, a power supply voltage, manufacturing variations in constituent elements thereof, or so on.
A common structure of the organic EL element is as shown in FIG. 1B. Because a transparent, electrically conductive film (ITO film) as a constituent member thereof has resistance as large as about 10 to 20 Ω/•, the same is used on the side of the anodic data lines SEGm where a large current does not flow (on the order of several hundred μA to 1 mA) while a resistance material such as Al is used on the side of the cathodic scanning lines COMn.
However, when causing all the elements in panel rows to emit light, a large current of several tens of mA flows in the direction of the grounding potential VG in the scanning lines COMn via the switching means SWc1-SWcn.
Even in the case of the scanning lines COMn using a resistance material such as an Al cathodic wiring, there flows a large current corresponding to the panel element connected thereto and a current value necessary for light emission, so that a voltage applied to the panel element PEm,n positioned at a more distal end in relation to the grounding potential VG becomes very high.
Assuming that resistance of the scanning lines COMn is Rm,n, a current flowing through the resistance is Icm,n, ON resistance of the switching means SWcn is SWrn, and a voltage applied to the organic EL element PEm,n when all the panel elements emit light is Vm,n as shown in FIG. 3, the following equation results:Vm,n=VC+SWrn*Ic1,n+R1,n*Ic1,n+R2,n*Ic2,n+ . . . +Rm,n*Icm,n
Herein, assuming that light-emitting display panel rows are 128 rows, resistance between the panel elements is Rm,n=r (Ω), and a current supplied to respective data lines SEGm is Im=i (A), the following equation results:
                              V                      m            ,            n                          =                ⁢                              V            C                    +                                    SW              rn                        *            128            ⁢                                                  ⁢            i                    +                      r            *            128            ⁢                                                  ⁢            i                    +                      r            *            127            ⁢                                                  ⁢            i                    +                      r            *            126            ⁢                                                  ⁢            i                    +          …          +          ri                                        =                ⁢                              V            C                    +                                    SW              rn                        *            128            ⁢                                                  ⁢            i                    +                      8256            ⁢                                                  ⁢            ri                              
That is, there occurs a potential as high as 8256 ri (V) owing to the resistance component of the scanning lines COMn.
Thus, since the farther from the grounding potential VG the EL element PEm,n is positioned at a distal end, the smaller a potential difference ΔV11 applied to the respective constant current sources 11 becomes, there have been cases where it becomes impossible to supply a constant current, depending on conditions such as dependency of the respective constant current sources 11 on output voltage, a constant current value, and a drive power supply voltage Vs.
Further, there is a tendency of an increase in the number of bits of a driver IC following an increase in the size of a panel screen, and such an increase in the number of the bits poses a problem in that not only deterioration in display unevenness, due to manufacturing variations, is brought about but also constant current characteristic dependent on resistance on the panel described above becomes susceptible to occurrence of faults.